A true hepa vac that's epa lead approved will run you about $600.00 and up wards. The paint shaver is great, but remember you still have to usually sand the clapboards using a random orbit sander after using the paint shaver.
When we completely strip all of the paint off of a building we prefer to use chemical stripping. Chemical stripping will remove all of the paint if you use the proper products. Either way you go it's time consuming and a labor intensive process to remove all of the paint from a building. Peel Away has several lines of paint strippers that work pretty well.

Yeah I'll have to sand it afterwards too. I already have a true hepa vac that meets EPA regulations. It was obnoxiously expensive for a vacuum.

I'm also restoring all of our windows, 30 double hung and 6 picture windows - so I'll be dealing with lead paint for a while. It'll be nice in a couple years when at least all the exterior lead paint is gone.

Don't burn the house down

The danger in using any form of heat gun to remove paint is that it may start wood dust (or paper, or anything flammable) on the inside of the wall smouldering, which will burst into flame after you've packed up and left the job for the day. The "infrared" paint remover site says it heats the paint to 400 degrees F; paper ignites at 454 degrees. "Infrared" is just heat, by the way.
Sanding the entire surface of a 3000 sq. ft. house is something to avoid. It would be easier, cheaper, and faster to put on entirely new clapboards. You would also see exactly what was beneath them and maybe needs fixing, and you would replace all the flashing and do it right for once and all. Paint the bottom narrow side of the clapboards a different color (while in a stack at ground level) so it will look like insanely detailed pinstriping when put up.

I haven't looked into the prices on clapboard - but I doubt from a DIY perspective that it would be cheaper. It is tempting so I'll do the math on it when I get closer to the project :-D

If you take a look over at Paint Talk one of the current threads called "Starting this Wednesday" is about a painting company doing a full RRP removal using the paint shaver ad infrared heat stripper. They don't like DIY's posting over there but you might get some insight on how it goes from the thread.